This invention relates to a method of assessing radar pulses in a passive receiving system to distinguish pulses due to a particular target from pulses due to other targets, from noise, interference etc. Received pulses may be assessed, without reference to pulse transmission information, in respect of a number of characteristics; for example, direction of reception (angle off boresight), power level, pulse width, pulse repetition interval (PRI), and also radio frequency, if an absolute measurement of frequency is available. Each of these characteristics, even in respect of a particular target, is subject to variation due to noise, interference etc., and significant tolerances have to be employed in the assessment of each pulse.
Conventional signal processing methods rely, in the case of active radars, on the known characteristics of the transmitted pulse. Such systems are primarily concerned with detection of target echoes among clutter. The receiver has a priori knowledge of the nature of the echo signal it is seeking to identify since it usually forms part of an active radar system, which has itself determined the transmitted pulse waveform. The task is, therefore, to pick out this known waveform from clutter and noise, and a comparison with the known pulse data is the obvious solution. This invention is directed to working in a much more demanding situation to identify a target which is emitting radar pulses that have not originated from the detecting radar. Thus, the target may well be a radar transmitter itself. In such a situation there is initially no foreknowledge of the received pulse waveform and it may have to be identified not so much from clutter as from other conflicting radar signals. Conventionally, passive, i.e. receive-only, radars have compared the value of the particular characteristic with preset limits and accepted or rejected the pulse accordingly. The tolerance band set for such a comparison leads to a significant probability of a wrong signal being accepted, with a consequent loss of ability to distinguish targets. This invention is directed to the statistical assessment of received pulses in respect of a number of characteristics by reference only to previously received pulses to establish a confidence value that a train of received pulses arises from a particular target.